A typical motor controller employs a rectifier part and an inverter part, where the rectifier converts multi-phase input voltages to a DC voltage. The inverter typically provides a bridge structure of diodes and transistors forming switching elements, where each of these switching elements includes a high-side and a low-side.
Gate drives are connected to turn on and off the switching elements, where each gate drive requires a voltage supply.
Low-cost commercial motor controllers often employ bootstrap-capacitor supplies of the gate drives for the high-side switching elements in the inverter. This is a cost-effective solution that has been on the market for years. Existing motor-controller products from Danfoss Drives A/S including the VLT® 2800 from 1998, the VLT® Automation Drive FC 30× from 2004 and the VLT® Micro Drive FC 51 from 2007, all utilize the bootstrap-capacitor principle. The method is to turn on the low-side switching element for a time period referred to as the bootstrap-charge time in a given phase for charging the bootstrap capacitor of this phase.
By turning on a low-side switching element the voltage reference of the corresponding high-side gate driver is being connected to the voltage reference of the low-side switching element, meaning that energy from a low-side DC voltage supply can be transferred to the corresponding bootstrap capacitor, typically, but not necessarily, via a current-limiting bootstrap resistor and a high-voltage diode.
Then, when the low-side switching element is off, the high-side bootstrap capacitor holds the needed energy for the high-side gate driver until the next on-state of the low-side switching element. This period is referred to as the hold-up time of the bootstrap capacitor in a given phase which together with the load current from the high-side gate driver and the minimum tolerated level of the high-side supply voltage, defines the required bootstrap capacitance.
The required bootstrap capacitance and the needed current-limiting-bootstrap resistor set the bootstrap-charge time, which should be as small as possible to have minimum impact on the motor-controller performance.
A desirable long hold-up time and a desirable short bootstrap-charge time essentially contradict each other.
Until the bootstrap capacitors are charged, the high-side switching elements will be inoperative (off), since the corresponding gate drives are without voltage supply.
Conventionally, the charging of the high-side gate drives is simple. At standstill, the low-side switching elements are turned on, and at the end of the bootstrap-charge time, the whole inverter may be started with all switching elements operative using normal PWM (Pulse Width Modulation), as for example described in the paper “Stator Flux Oriented Asynchronous Vector Modulation for AC-Drives” presented at the international conference PESC'90. If the drive is not started within the hold-up time of any of the bootstrap capacitors, a new bootstrap-charge period is needed, before normal PWM can be entered.
After a bootstrap when all switching have become operative is entered, the system is in the normal PWM mode where is well known that the bootstrap recharging must run continuously to keep all switching elements operative. The normal PWM must be designed not to violate the hold-up time as given by the bootstrap-capacitor design, which is a constraint when using low-cost bootstrap supplies. A literature example of this is U.S. Pat. No. 6,570,353 B2, describing a starting algorithm of a PM-motor for maintaining operative switching elements after the bootstrap supplies initially were charged by a bootstrap sequence at standstill.
Recharging of the bootstrap supplies is not a major problem at low-speed operation, or start-up, of most motor controllers. For a 3-phase motor controller as in U.S. Pat. No. 6,570,353 B2, all low-side switches will be modulated in every switching period giving short time periods in the off state only at low speed. Hence, all bootstrap capacitors are continuously recharged sufficiently. Problems however may occur at high speed levels, where the well-known overmodulation technique frequently is used to generate a sufficient level of output voltage for the load.
It may happen in a high speed region that in a particular interval in time one of the low-side switches is modulated in two or more subsequent switching periods, with the consequence of an enlarged period of time where the corresponding bootstrap supply is not charged. This time period may risk approaching the hold-up time for the bootstrap supply.
In another high speed region it may happen in a particular interval in time that two low-side switches are modulated in two or more subsequent switching periods, with the consequence of an enlarged period of time where the corresponding two bootstrap supplies are not charged. Again, this time period may challenge the hold-up time of the bootstrap supplies.
In some drives the low-side switching elements are turned on and off (PWM modulated) within the bootstrap-charge time to limit the stress on the motor controller in general, and also to limit the stress on the motor, if it is spinning and energized (magnetized), in this situation operating as a generator inducing voltages on the output terminals of the motor controller, thus inducing the back-EMFs.
Even with a PWM modulated bootstrap-charge period, there may still be induced an over-voltage in the motor controller and/or an over-current in both the motor controller and the motor. For permanent magnet motors this could be destructive. In worst case the bootstrap sequence is equal to a 3-phase short-circuit condition of the spinning motor.
Those skilled in the art may recognize this problem as being highly relevant for a spinning permanent-magnet motor.